The present invention relates to diaphragms useful in electrolytic cells utilized for the electrolysis of solutions, especially useful in electrolytic cells utilized for the electrolysis of aqueous alkali metal halide solutions, e.g., sodium chloride brine. More particularly, this invention relates to non-asbestos diaphragms useful in such cells.
Alkali metal halide brines, such as sodium chloride brines and potassium chloride brines, may be electrolyzed in a diaphragm-type cell to yield chlorine, hydrogen, and aqueous alkali metal hydroxide. A diaphragm-type cell (hereinafter referred to as a diaphragm cell) includes a diaphragm dividing the cell into an anolyte compartment with an anode therein and a catholyte compartment with a cathode therein. In such a diaphragm cell, brine is fed to the anolyte compartment and a halogen, e.g., chlorine is evolved at the anode. Electrolyte percolates from the anolyte compartment through the electrolyte permeable diaphragm to the catholyte compartment where hydroxyl ions and hydrogen gas are evolved at the cathode. Optionally, an oxygen cathode may be used to reduce the cell voltage and consequently eliminate the production of hydrogen. The product in the catholyte compartment is commonly referred to as cell liquor and contains a mixture of alkali metal hydroxide and alkali metal halide.
The majority of commercial diaphragms have been comprised of fibrous asbestos. These diaphragms can be formed by vacuum-deposition of asbestos from a slurry onto an electrolyte permeable cathode. Unfortunately, asbestos diaphragms have suffered from short lifetimes of about 6 to 8 months. The development of dimensionally stable metal anodes led to a search for diaphragms with longer life. Asbestos diaphragms have been strengthened by vacuum-depositing the asbestos from an aqueous slurry containing sodium hydroxide and baking the diaphragm at between 110.degree.-280.degree. C. for at least 2 hours as described in U.S. Patent No. 3,991,251. Various polymer resins have also been included in a resin-asbestos diaphragm as shown by, e.g., U.S. Pat. Nos. 3,694,281; 3,723,264; 3,853,721; and 4,093,533.
Microporous separators without asbestos have been produced by sintering materials such as polytetrafluoroethylene (PTFE) and a particulate pore forming additive followed by subsequent removal of the additive, as shown by, e.g., U.S. Pat. Nos. 3,930,979; 4,098,672; and 4,250,002. While such microporous separators have a long service life, they have been produced in the form of sheets and are not easily utilized in electrolytic cells having complex non-planar electrode geometries, such as diaphragm cells with fingered anodes and cathodes. The need remains for an improved diaphragm, preferably a non-asbestos diaphragm, which can be used in cells having such non-planar electrodes.
Besides a desire to eliminate asbestos, the catholyte or cell liquor recovered after electrolysis of sodium chloride brine in typical asbestos diaphragm cells generally has a sodium hydroxide concentration of about 10 to 12 weight percent and a sodium chloride concentration of about 12 to 16 weight percent. Cell liquor must then be processed to separate sodium chloride and concentrate the sodium hydroxide to a generally desired level of about 50 weight percent. This processing is energy intensive. Production of cell liquor having a greater concentration of sodium hydroxide is desirable. Yet, diaphragm cells with asbestos-type diaphragms have difficulties in achieving a catholyte with a sodium hydroxide strength in excess of 12 or 13 weight percent under economically tolerable current efficiencies.
Therefore, it is highly desirable to have a non-asbestos diaphragm which can be readily utilized in diaphragm cell equipment currently in operation. Further, it is desirable to have a diaphragm which allows brine electrolysis to proceed so as to obtain sodium hydroxide concentrations within the catholyte from about 12 to 20 weight percent or more, preferably greater than about 15 weight percent at current efficiencies of greater than about 88%. Finally, it is desirable to have a non-asbestos diaphragm capable of a reduced operational voltage in comparison to previous asbestos-containing diaphragms.